U.S. patent application number 12/256289 was filed with the patent office on 2009-02-12 for fiber-optic current sensor with polarimetric detection scheme.
Invention is credited to Klaus Bohnert, Hubert Brandle, Andreas Frank.
Application Number | 20090039866 12/256289 |
Document ID | / |
Family ID | 37635892 |
Filed Date | 2009-02-12 |
United States Patent
Application |
20090039866 |
Kind Code |
A1 |
Bohnert; Klaus ; et
al. |
February 12, 2009 |
Fiber-Optic Current Sensor With Polarimetric Detection Scheme
Abstract
The current in a conductor is measured by exploiting the Faraday
effect in a sensing fiber. The light returning from the sensing
fiber is split into at least two parts, at least one of which is
analyzed by a first circular analyzer for generating a first
signal. A second part may e.g. be analyzed by a second circular
analyzer, and a third part may be analyzed by a linear analyzer. By
combining the signals obtained in this way, the current induced
phase delay in the returning light can be measured efficiently and
accurately.
Inventors: |
Bohnert; Klaus;
(Oberrohrdorf, CH) ; Frank; Andreas; (Zurich,
CH) ; Brandle; Hubert; (Oberengstringen, CH) |
Correspondence
Address: |
ST. ONGE STEWARD JOHNSTON & REENS, LLC
986 BEDFORD STREET
STAMFORD
CT
06905-5619
US
|
Family ID: |
37635892 |
Appl. No.: |
12/256289 |
Filed: |
October 22, 2008 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
PCT/CH2006/000227 |
Apr 25, 2006 |
|
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|
12256289 |
|
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Current U.S.
Class: |
324/97 |
Current CPC
Class: |
G01R 15/246
20130101 |
Class at
Publication: |
324/97 |
International
Class: |
G01R 13/40 20060101
G01R013/40 |
Claims
1. A method for measuring a current comprising the steps of sending
left and right circularly polarized light waves at least once along
a sensing fiber Overextending around said current, passing said
right and left circularly polarized light waves through an optical
retarder for generating two orthogonally linearly polarized
returning light waves, sending a first part of said returning light
waves to a first detector, a second part of said returning light
waves to a second detector, characterized by the step of sending a
third part of said returning light waves to a third detector,
wherein said first part of said returning light is passed through a
first circular analyzer before impinging on said first detector,
wherein said second part of said returning light waves is passed
through a second circular analyzer before impinging on said second
detector and having a polarization opposite to said first circular
analyzer and wherein said third part of said returning light waves
is passed through a linear analyzer before impinging on said third
detector, wherein said first detector generates a signal I.sub.+
and said second detector generates a signal I.sub.- and said third
detector generates a signal I.sub.L, and knowledge of the three
signals I.sub.+, I.sub.-, I.sub.L allows to obtain the current even
if the values for fringe visibility V and arriving optical power
I.sub.0 are not known in advance.
2. The method of claim 1, said method further comprising the step
of calculating S=(I.sub.+-I.sub.-)/(I.sub.++I.sub.-) and the step
of calculating I.sub.L/(I.sub.++I.sub.-).
3. The method of claim 2, wherein S=Vsin(.DELTA..phi.), with
.DELTA..phi. being a phase shift introduced by Faraday-effect
between said circularly polarized light waves in said sensing fiber
and V describing an interference fringe visibility, wherein said
method comprises the step of calculating .DELTA..phi. from said
signals I.sub.L, I.sub.+ and I.sub.-.
4. The method of claim 3, wherein the signal
S'=I.sub.L/(I.sub.++I.sub.-), is used to compensate the signal S
for variations in fringe visibility V.
5. The method of claim 1, further comprising the step of estimating
a temperature at said retarder, or a correction factor depending on
said temperature, from said signals.
6. The method of claim 5, wherein said retarder has a phase shift
not equal to 90.degree. and/or wherein said temperature and/or said
correction factor is estimated from I.sub.L/(I.sub.++I.sub.-),
where I.sub.L is a signal generated by said third detector.
7. The method of claim 1, wherein said retarder has a phase shift
equal to 90.degree.+.epsilon., with .epsilon. being a non-zero
deviation, wherein said deviation is chosen such that cos
.epsilon..DELTA..phi. becomes independent of temperature, with
.DELTA..phi.being a phase shift introduced between the right and
left circularly polarized light waves in said fibers.
8. The method of claim 6, wherein a deviation .epsilon. of the
retarder from .pi./2 is present and the signal
I.sub.L'=I.sub.L/(I.sub.++I.sub.-)=(1+cos.sup.2 .epsilon.), is
calibrated so that it becomes a measure for the temperature.
9. The method of claim 8, wherein the signal I.sub.L is determined
at the zero-crossings of an alternating current.
10. The method of claim 1, wherein said first and/or second
circular analyzer is comprised of a quarter wave retarder and a
linear analyzer.
11. The method of claim 1, wherein said returning light waves are
passed through an integrated optics device, comprising at least one
splitter for generating said first and said second part, and in
particular at least two beam splitters.
12. The method of claim 11, wherein said integrated optics device
comprises at least one integrated quarter-wave retarder cooperating
with at least one external linear polarizer for forming said
circular analyzer.
13. The method of claim 1, wherein said third part of said
returning light waves is not passed through a linear analyzer
before impinging on said third detector, and the signal I.sub.L
generated at said third detector equals I.sub.0, which is
proportional to the arriving optical power.
14. The method of claim 1, wherein for small currents
I.sub.+=I.sub.0(1+V.DELTA..phi.), I.sub.-=I.sub.0(1-V.DELTA..phi.),
and for alternating currents, the ac and dc components of the
signals I.sub.+ and I.sub.- are extracted in a signal processors,
and the quotient of the ac and dc components gives a signal
V.DELTA..phi. proportional to the wave form of the current and
independent of variations in the light intensity.
15. The method of claim 1, wherein no ac phase modulators are
present.
16. A method for measuring a current comprising the steps of
sending left and right circularly polarized light waves at least
once along a sensing fiber extending around said current, passing
said right and left circularly polarized light waves through an
optical retarder for generating two orthogonally linearly polarized
returning light waves, sending a first part of said returning light
waves to a first detector, characterized by the step of passing
said first part of said returning light through a first circular
analyzer before impinging on said first detector, passing a further
part of said returning light waves through a linear analyzer before
impinging on a further detector, wherein said first detector
generates a signal I.sub.+ or I.sub.- and said further detector
generates a signal I.sub.L, and knowledge of the two signals
I.sub.+ or I.sub.- and I.sub.L allows to eliminate an arriving
optical power I.sub.0 and to calculate the phase shift .DELTA..phi.
assuming that a fringe visibility V is known or has been measured
in different manner.
17. The method of claim 16 comprising the step of dividing I.sub.+
by I.sub.L I.sub.+/I.sub.L=(1+Vsin(.DELTA..phi.))/(1+V).
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] The present application is a continuation of pending
International patent application PCT/CH2006/000227 filed on Apr.
25, 2006, which designates the United States and the content of
which is incorporated herein by reference.
FIELD OF THE INVENTION
[0002] The invention relates to the field of measurement of
electrical currents using the propagation of light waves in an
optical fiber under influence of the Faraday effect.
BACKGROUND OF THE INVENTION
[0003] The measurement of currents using light waves in an optical
fiber wound around a conductor has e.g. been described in EP 856
737.
[0004] In this device, two orthogonal, linearly polarized light
waves are sent through an electro-optic phase modulator for
introducing an ac phase modulation and then over a polarization
maintaining fiber (PMF) to a site of measurement, where they are
converted by a retarder to two circularly polarized light waves of
opposite orientation. These circularly polarized light waves pass
through a measuring fiber wound around a conductor. At the end of
the measuring fiber, a reflector sends the light waves back to the
retarder, where they are converted back to two linearly polarized
light waves. The returning light from the retarder is separated
from the original light in a beam splitter and sent to a
detector.
[0005] Alternatively, devices based on fiber gyro modules can be
used.
SUMMARY OF THE INVENTION
[0006] It is an object of the present invention to provide method
that allows to measure the current accurately and with simple
means.
[0007] This object is achieved by the method of claim 1.
Accordingly, the returning light is split into two or more parts. A
first part is passed through a circular analyzer and then measured
by a first detector. A second part fed to a second detector. The
second part of the light can be analyzed in a manner different from
the first part, e.g. by a circular analyzer of opposite direction,
or by a linear analyzer, or it can be passed directly (without
passing through any polarizer) to the second detector. Hence, the
method allows to determine two or more characteristic parameters of
the returning light, which allows to obtain a more accurate result
easily. No ac phase modulators are required.
[0008] The term "analyzer" designates an optical device that lets a
given polarization pass to the detector while blocking the opposite
or perpendicular polarization.
BRIEF DESCRIPTION OF THE DRAWINGS
[0009] The invention will be better understood and objects other
than those set forth above will become apparent when consideration
is given to the following detailed description thereof. Such
description makes reference to the annexed drawings, wherein:
[0010] FIG. 1 is a first set-up for carrying out the method
according to the present invention,
[0011] FIG. 2 is a second set-up for carrying out the method
according to the present invention, and
[0012] FIG. 3 is a third set-up for carrying out the method
according to the present invention.
DETAILED DESCRIPTION OF THE INVENTION
[0013] In the set-up of FIG. 1, light of a low-coherent broad-band
source 1 (e.g. a superluminescent diode (SLD), a laser diode
operated below threshold, an edge emitting LED (ELED), or a doped
fiber light source), with a spectral width of e.g. some 10 nm,
passes through a depolarizer 2 (e.g. a Lyot-type fiber polarizer as
described in W. K. Burns, Degree of polarization in the Lyot
depolarizer, Journal of Lightwave Technology LT-1, 475 (1983)). The
light is subsequently polarized in a fiber polarizer 3. The
depolarizer 2 can be omitted if an unpolarized light source is
used.
[0014] The polarized light from polarizer 3 enters the polarization
maintaining fiber (PMF) pigtail 4 of an integrated-optics beam
splitter device 5 with its polarization direction parallel to the
fast or slow axis of the PMF. The fiber is pigtailed to device 5
with it birefringent axes at +45.degree. or -450 to the flat chip
surface. Alternatively, fiber 4 is aligned at 0.degree. or
90.degree. to the chip surface. The waveguides of device 5 have
preferably low birefringence.
[0015] The light exits from device 5 to a transmission fiber 6,
which is a PMF with its birefringent axes aligned at +450 or -45 in
respect to the axes of PMF pigtail 4. Hence, as a result, two
orthogonally polarized light waves are launched into transmission
fiber 6. They arrive at a quarter-wave retarder 7 at a first end of
a sensing fiber 8. Retarder 7 converts the two linearly polarized
waves to first left and right circularly polarized light waves,
which propagate in sensing fiber 8 and are reflected at an end
thereof by a mirror 9, thereby generating second right and left
polarized light waves propagating back to retarder 7.
[0016] Sensing fiber 8 is wound at least once around a conductor
10. The magnetic field of a current through conductor 10 gives rise
to a Faraday effect in sensing fiber 8, which in turn creates a
phase shift .DELTA..phi. between the right and left circularly
polarized light waves returning to retarder 7.
[0017] The sensing fiber is advantageously prepared as described in
EP 856 737 (a thermally annealed fiber, in case of small coil
diameters of less than 300-500 mm) or packaged as described in EP 1
512 981 (in case of relatively large coil diameters and when
flexible coils are desired).
[0018] In retarder 7, the second right and left linearly polarized
light waves are converted to returning linear light waves oriented
along the main axes of PMF 6. The polarization directions of the
returning light waves are swapped with respects to the forward
propagating waves in PMF 6.
[0019] In device 5, the returning light is split into several parts
at three beam splitters 12, 13, 14. Three of these parts are used
in subsequently described measurements.
[0020] A first part arrives at a first circular analyzer formed by
a first quarter-wave retarder 15a and a first linear polarizer 15b,
where quarter-wave retarder 15a has its fast and slow axes aligned
parallel or orthogonally to the fast and slow axes of fiber 6, and
polarizer 15b stands approximately under 450 thereto. The light
exiting from first circular analyzer 15a, 15b, which is e.g. a left
circular analyzer, is analyzed by a first light detector 16.
[0021] A second part of the returning light arrives at a second
circular analyzer formed by a second quarter-wave retarder 17a and
a second linear polarizer 17b, where quarter-wave retarder 17a has
its fast and slow axes aligned parallel or orthogonally to the fast
and slow axes of fiber 6, and polarizer 15b stands approximately
under -45.degree. thereto. The light exiting from second circular
polarizer 17a, 17b, which is polarizing opposite to the first
polarizer, is analyzed by a second light detector 18.
[0022] A third part of the returning light arrives at a linear
polarizer 19 arranged at about 45.degree. to the axes of PMF 6. The
light from polarizer 19 is measured in a detector 20.
[0023] The signals measured by the detectors 16, 18 and 20 are fed
to a signal processor 21.
[0024] In the following, the signals from the various detectors 16,
18, 20 are calculated.
[0025] The signals I.sub.+ and I.sub.- from first detector 16 and
second detector 18 are
I.sub..+-.=I.sub.0(1.+-.Vsin(.DELTA..phi.)), (1) [0026] where
I.sub.0 is proportional to the optical power arriving at the
retarders/polarizers 15a, 17a and 19 and V is the interference
fringe visibility (V is equal to unity at ideal conditions of
interference). For simplicity, it is assumed that the splitting
ratios of the beam splitters 12, 13, 14 are such that the optical
power is the same in all exit channels. The phase shift
.DELTA..phi. is given by
[0026] .DELTA..phi.=4K.sub.VNI, (2) [0027] with K.sub.V being the
Verdet constant (e.g. 1.0 .mu.rad/A at 1310 nm), N is the number of
sensing fiber loops, and I is the current. At 1310 nm and N=1,
.DELTA..phi.=.+-..pi./2 corresponds to currents of about .+-.390
kA.
[0028] The retarders 15a, 17a introduce an approximately 90.degree.
phase offset between the two returning linear light waves. The two
waves are brought to interference at the analyzers 15b, 17b. Due to
the 90.degree. phase shift, the interference signals after
analyzers 15b and 17b vary in good approximation linearly with the
magneto-optic phase shift (current), as long as the phase shift
(current) is sufficiently small. At phase shifts .DELTA..phi.
approaching .+-..pi./2 or .+-.90.degree., a linearization of the
sinusoidal transfer function in the signal processor 21 is
necessary.
[0029] At small currents equation (1) becomes, in
approximation,
I.sub..+-.=I.sub.0(1.+-.V.DELTA..phi.). (3)
[0030] The signal I3 from detector 20 is as follows:
I.sub.L=I.sub.0(1+V cos .DELTA..phi.). (3a)
[0031] For small .DELTA..phi., I.sub.L can be approximated by
I.sub.L=I.sub.0(1+V). (4)
[0032] For alternating currents, .DELTA..phi. is equal to
.DELTA..phi.=.DELTA..phi..sub.0sin(.omega.t). (5) [0033] where
.DELTA..phi.0 is the amplitude of the magneto-optic phase
modulation, .omega. is the current angular frequency and t is
time.
[0034] Hence, the ac and dc components of Eq. (3) are
I.sub.ac=I.sub.0V.DELTA..phi. (6)
I.sub.dc=I.sub.0. (6a)
[0035] These values can be extracted in the signal processor 21.
Dividing (6) by (6a) gives a signal (V..DELTA..phi.) which is
proportional to the wave form of the current and independent of
variations in the light intensity (for example due to source power
variations or varying optical loss).
[0036] However, this method is restricted to ac currents only, and
it is of limited accuracy. In the following, improved methods
described.
[0037] In a first advantageous embodiment, the system of equations
(1) and (4) is solved to obtain the value of .DELTA..phi.. Even
though these equations have three unknowns (I.sub.0, V and
.DELTA..phi.), this is possible because there are three independent
equations.
[0038] We can e.g. calculate
S = ( + - - ) / ( ++ - ) ( 7 ) = V sin ( .DELTA..PHI. ) ( 8 )
##EQU00001##
[0039] In the linear approximation of equation (8), we have
S=V.DELTA..phi. (8a)
[0040] On the other hand, in linear approximation,
S ' = I L / ( + + - ) = ( 1 + V ) or ( 9 ) V = S ' - 1 ( 9 a )
##EQU00002##
[0041] Hence, we have
.DELTA..phi.=S/V=S/(S'-1). (10)
[0042] In other words, the signal S' can be used to compensate the
signal S for variations in fringe visibility V.
[0043] The techniques shown here also allow for a compensation of
the temperature dependence of the Faraday effect. Two mechanisms
can be used, namely an "intrinsic compensation" and an "extrinsic
compensation", both of which are explained in the following.
[0044] Intrinsic compensation:
[0045] The retardation p of the retarder 7 at the fiber coil
commonly varies somewhat with temperature. For example, the
retardation may decrease at a rate of
(1/.rho.)(.delta..rho./.delta.T)=-2.210.sup.-4.degree. C..sup.-1
(see K. Bohnert et al., J. Lightwave Technology 20, 267-276, 2002).
This affects the effective fringe visibility and hence the
relationships for I.sub..+-. and I.sub.L. If .epsilon. is the
deviation of retarder 7 from .pi./2 one obtains from a Jones matrix
description of the light propagation
I.sub..+-.=I.sub.0(1.+-.cos .epsilon.sin .DELTA..phi.) (11)
[0046] Here, ideal conditions of interference are assumed, i.e. V=1
for .epsilon.=0. For and small .DELTA..phi., eq. (11) becomes
I.sub..+-.=I.sub.0(1.+-.cos .epsilon..DELTA..phi.). (12)
[0047] The variation of s with temperature can be used to
intrinsically compensate for the temperature dependence of the
Verdet constant. The Verdet constant K.sub.V, and hence
.DELTA..phi. at a given current, increase with temperature at a
rate of 0.710.sup.-4.degree. C..sup.-1. If the retarder is prepared
with a room temperature retardation of about 77.degree. (i.e.
.epsilon.=-13), the increase in .DELTA..phi. is just balanced by
the decrease in the cos .epsilon. term--i.e. the product
.DELTA..phi..sub.comp=cos .epsilon..DELTA..phi. in eq. (12) becomes
independent of the temperature (assuming that the retarder and the
sensing fiber have equal temperature).
[0048] Note: With a temperature dependence of .rho. given above,
.rho. decreases from about 78.degree. (.epsilon.=-12.degree.) to
76.degree. (.epsilon.=-14.degree.) if the temperature rises from
-40.degree. C. to 80.degree. C. Hence, the (cos .epsilon.)-term
decreases by a factor of 1.008, while the Verdet constant K.sub.V
increases by about the same factor.
[0049] In K. Bohnert et al., J. Lightwave Technology 20, 267-276,
2002 and EP 1 115 000 an interferometric detection concept was used
to measure the Faraday effect. Here, the retarder was also employed
for intrinsic temperature compensation. In this case, the variation
in the retardation on the recorved phase shift is used. For
compensation the retardation must be set to about 100.degree. if
the same type of retarder is used.
[0050] Extracting the Temperature Signal from I.sub.L:
[0051] Alternatively, the temperature can be extracted from the
signal I.sub.L at detector 20, again using a retarder at the fiber
coil differing from .pi./2. Assuming V=1, one obtains
I.sub.L=I.sub.0[(1+cos.sup.2 .epsilon.)cos.sup.2(.DELTA..phi./2)].
(13)
Furthermore, I.sub..+-. are again
I.sub..+-.=I.sub.0(1.+-.cos .epsilon.sin .DELTA..phi.). (14)
At sufficiently small currents, eq. (13) becomes
I.sub.L=I.sub.0(1+cos.sup.2 .epsilon.). (15)
Dividing I.sub.L by the sum I.sub.++I.sub.- yields
I'.sub.L=I.sub.L/(I.sub.++I.sub.-)=(1+cos.sup.2 .epsilon.) (16)
[0052] Since .epsilon. varies with temperature, the signal I'L can
be calibrated so that it becomes a measure for the temperature. For
an unambiguous result .epsilon. must be chosen such that the
retardation stays smaller than 90.degree. or lager than 90.degree.
over the temperature range of operation. If the small current
approximation is not valid, the signal IL of eq. (15) can be
determined at the zero-crossings of an alternating current, where
the term in cos.sup.2(.DELTA..phi./2) in (13) vanishes.
[0053] The ratio (I.sub.+-I.sub.-)/(I.sub.++I.sub.-) gives, for
small currents:
(I.sub.+-I.sub.-)/(I.sub.++I.sub.-)=cos .epsilon..DELTA..phi.
((17)
[0054] The term cos .epsilon. in (17) is obtained from (16), i.e.
.DELTA..phi. is expressed in terms for IL, I+ and I-.
[0055] .DELTA..phi. is then temperature compensated in the signal
processor using the temperature extracted from (16).
[0056] Further General Notes:
[0057] As described above, knowledge of the three signals I.sub.+,
I.sub.- and I.sub.L allows to obtain very accurate results for the
phase shift .DELTA..phi. and the current even if the values for V
and I.sub.0 not known in advance. It must be noted, however, that
even the knowledge of two of the three signals I.sub.+, I.sub.- and
I.sub.L is advantageous over the prior art.
[0058] For example, FIG. 2 shows a device that measures I.sub.+ and
I.sub.- only. In this case, the calculation of S according to
equation (8) allows to calculate .DELTA..phi. under the assumption
that V is known or has been measured in different manner.
[0059] On the other hand, a knowledge of I.sub.+ and I.sub.L (or,
equivalently, I.sub.- and I.sub.L) allows again to eliminate 10,
e.g. by dividing I.sub.+ by I.sub.L
I.sub.+/I.sub.L=(1+Vsin(.DELTA..phi.))/(1+V). (18)
[0060] Again, this allows a calculation of the phase shift
.DELTA..phi. assuming that V is known or has been measured in
different manner.
[0061] FIG. 3 shows a device suitable for carrying out this third
embodiment of the invention.
[0062] FIG. 3 also shows an alternative to using the external
retarder 15a (or 17a), namely by integrating a birefringent
waveguide 22 with a phase shift of .pi./2, the light from which is
directly fed to detector 16. The .pi./2 phase shift could also be
in the entrance branch of the beam splitter for generating I.sub.+
and I.sub.- after the analyzers.
[0063] It is known that UV exposure can alter the birefringence or
introduce birefringence in optical waveguides, see e.g. Meyer,
P.-A. Nicati, P. A. Robert, D. Varelas, H.-G. Limberger, and R. P.
Salathe, Reversibility of photoinduced birefringence in
ultralow-birefringence fibers, Optics Letters, 21, 1661 (1996), or
T. Erdogan and V. Mizrahi, Characterization of UV-induced
birefringence in "photosensitive Ge-doped silica optical fibers",
Journal of the Optical Society of America B11, 2100 (1994). In the
arrangement of FIG. 3, the waveguide 22 has been irradiated by UV
radiation for generating a birefringence that introduces a .pi./2
phase shift.
[0064] In principle, the birefringence in the two legs of the
splitter causing a .pi./2 phase retardation can also be achieved by
other means, e.g. a somewhat non-circular geometry of the
waveguide, a thinner surface layer above the waveguide, or built-in
stress.
[0065] In the embodiments above, the signal I.sub.L was generated
by sending the returning light through linear polarizer 19 before
measuring it with a detector 20. It must be noted that polarizer 19
can also be omitted, in which case equation (4) would have to be
replaced by
I.sub.L=I.sub.0. (19)
[0066] Again, equation (12) can be combined with the expressions
for I+ and/or I- of equations (1) or (3) for calculating
.DELTA..phi. and/or V.
[0067] Instead of using an integrated beam splitter, such as device
5, the present method can also be carried out by means of discrete
beam splitters, retarders and polarizers.
[0068] The invention has been described in reference to a sensing
fiber with a mirror. However, it can also be applied to set-ups
where the circular light waves pass through the sensing fiber only
once.
* * * * *